FWIW, my DS who is almost 14 months sounds exactly the same as your daughter- tonnes of babble, noisy all the time, but no words, and I'm not sure 'mamama' and 'dadada' mean the right things either!

My SIL is a speech/hearing pathologist and she seems to know about these things, and she said 18 months is quite normal for first words. So I'm not too worried. Most of the other kids in mothers group don't have any words yet either.